1. Field of the Invention
The invention is concerned with infrared transmission media in infrared radiation detection and transmission apparatus.
2. Description of the Prior Art
Infrared radiation detection devices have found commercial and military application, for example, in thermometers, alarm systems, night vision systems, and anti-aircraft missiles. In the latter application sensitive components of the infrared detection device are typically shielded by a window made of a material which not only permits infrared radiation to pass freely but which is also resistant to deterioration or breakage due to thermal shock resulting from atmospheric friction or rain. U.S. Pat. No. 3,911,275, issued to W. H. Dumbaugh, on Oct. 7, 1975, discloses a glass composed of GeO.sub.2, La.sub.2 O.sub.3, Ta.sub.2 O.sub.5, and ZnO for use as an infrared transparent window in infrared detection systems.
Materials which are infrared transparent and resistant to thermal shock are also applicable in infrared transmission apparatus such as high power lasers which can be used, e.g., for separating gaseous isotopes, for atomic fusion, for military application, and for medical application in microsurgery. In the latter case the window may take the form of an optical fiber which may be instrumental, e.g., in guiding infrared radiation so as to cauterize abdominal ulcers.
According to Frances M. Lussier, "Guide to ir-transmissive Materials," Laser Focus, December 1976, pages 47-50, thermal shock resistance can be measured by a figure of merit which is inversely dependent on thermal expansion. Consequently, small thermal expansion is desirable in infrared window material. Furthermore, to minimize the tendency towards internal fracture, thermal expansion should desirably be isotropic.